HE TO IDENTIFY THE PRESENCE OF ANCIENT GROUNDWATER IN RIVER SYSTEMS

The location and amount of discharge from deeply-sourced, regional groundwater systems has important implications for a large number of practical problems in Earth science including: migration of deeply sourced contaminants, proper management of water resources at the watershed scale and prediction of hydroecological response to prolonged climatic shifts. Most current studies either explicitly ignore the role of deep groundwater discharge in shallow alluvial systems, or use methods for estimating groundwater discharge which are incapable of identifying very old, deeply-sourced groundwater circulation. Here we use terrigenic ⁴He measured in river water and shallow alluvial aquifers, to provide unequivocal evidence of the discharge of groundwater with residence time in excess of thousands of years in several alluvial systems across Australia. By combining ⁴He with other common environmental tracers, we can constrain the amount of regional groundwater discharge to river systems, and the fraction of the total groundwater discharge comprised of regional groundwater. Groundwater with apparent age in excess of 1000 years comprises up to 10-100% of the total groundwater discharge in large river systems at the ~100 km reach scale. Our evidence indicates that over a range of geologic, climatic and hydrologic systems, regional groundwater discharge is obviously apparent, and plays an important role in shallow alluvial systems. Proper management and protection of these alluvial systems is only possible when the contribution of groundwater derived from regional flow paths is properly understood.